Absorbent article

ABSTRACT

An absorbent article, such as a diaper, diaper pants, sanitary towel or incontinence protector, with a vapor-permeable backing layer, including a liquid-permeable cover layer intended to be directed toward the user during use, a liquid-impermeable but vapor-permeable backing layer intended to be directed away from the user during use, and an absorbent core between the cover layer and the backing layer. The article has a longitudinal direction, a transverse direction, two substantially longitudinal side edges, a substantially transverse front edge, a substantially transverse rear edge, a substantially longitudinal midline, and a front part and a rear part on each side of a center line. The midline and center line intersect one another at a point of intersection. The absorbent article includes at least one monovalent salt of the form X + Y −  in a quantity of 1-75% by weight calculated on the basis of the weight of the core.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is a §371 National Stage Application of PCTInternational Application No. PCT/SE2006/050488 filed Nov. 17, 2006.

FIELD OF THE INVENTION

The present invention relates to an absorbent article, such as a diaper,diaper pants, sanitary towel or incontinence protector, with avapour-permeable backing layer, comprising at least one monovalent saltof the form X⁺Y⁻.

BACKGROUND OF THE INVENTION

Microorganisms commonly appear in absorbent articles during use.Microorganisms are introduced into absorbent articles via body fluids,such as urine or menstrual fluid, or by contact with the skin. Thegrowth of bacteria in absorbent articles may be considerable during thetime these articles are in use. With a high number of microorganismsthere is an increased risk of urinary tract infections and skinirritation, and odors are also to a large extent associated with thepresence of bacteria.

The bacteria are often ones that commonly occur in the individual. Thereis normally an ecological balance between different microorganisms onthe skin and mucous membranes, and the normal microbial flora isimportant for ensuring that undesired microorganisms do not have thechance to start growing. Sometimes this balance can be disturbed, suchthat potential pathogenic microorganisms may establish themselves, growand then give rise to infections, for example in connection withmedication, poor hygiene, poor immune defense, excessive hygiene, skinchanges, changes in the mucous membranes, and long-term use of absorbentarticles.

The use of absorbent articles over a long period of time without regularchanging or with poor hygiene, combined with an impaired immune defense,can increase the growth and spread of microorganisms. Moreover,tight-fitting clothes and underwear can increase the risk of growth ofmicroorganisms. With an increased number of undesired microorganisms inan individual or in an absorbent article, there is an increased risk ofmicrobial imbalance and of infections.

A natural part of the prevention of urogenital infections lies inimproved personal hygiene. However, it may not be appropriate to washthe genital area with strong soap or bactericidal agents, and it maytherefore be difficult for the individual to reduce the risk ofinfection to a suitable level using conventional means. Excessivelythorough washing with strong cleaning agents also disturbs the normalflora of desired bacteria that help protect us against the undesiredbacteria. It can also sometimes be difficult to change absorbentarticles. Long-term use can also increase the risk of infections ifmicroorganisms continue to grow within the article. Occlusion andtemperature favor the growth of microorganisms in the article and on theuser's skin and mucous membranes.

Resulting infections have traditionally been treated with conventionalantibiotics. However, repeated treatment with antibiotics can lead tothe development of resistant bacterial strains, which can make futuretreatment of infections difficult. A further problem with antibiotictreatment is that many individuals are hypersensitive to antibiotics.Moreover, antibiotics may be damaging to the skin and to the vaginalflora in women, with the result that the infection may reappear. The useof antibiotics can also have the disadvantage of disturbing anddestroying the bacteria in the normal flora.

Previous attempts have been made to reduce the growth of microorganismsand bacteria in absorbent articles by addition of bacteriostatic agentsand lowering of the pH.

It has also been proposed to use lactic acid bacteria on account oftheir inhibiting effect on pathogens. The use of lactic acid bacteriahas been shown to reduce the occurrence of infection on both skin andmucous membranes.

WO 2004/105822 describes the use of absorbent articles provided withbacteria that produce lactic acid. The bacteria cells are transferred tothe user's skin and reduce the risk of microbial infection in the user'surogenital region and on the user's skin.

EP 1 032 434 B1 describes an absorbent article that contains lactic acidbacteria. The bacteria are arranged to be transferred to the user's skinin order to worsen the conditions for undesired microorganisms.

EP 510 619 cites a number of agents that prevent the growth of bacteria,for example chlorhexidine, quaternary ammonium compounds, copper salts,chelating agents, parabens, chitin and pH buffers.

Several examples of antimicrobial agents are cited in US 2004/0180093,for example silver compounds, copper compounds, and zinc compoundscontained in a polymer composition. Silver, copper and zinc ions haveantibacterial properties.

U.S. Pat. No. 4,883,478 describes the preparation of an absorbentcomposition containing saccharide. Monosaccharide and/or disaccharide ismixed with superabsorbent to give a homogeneous paste. The absorbentmaterial can be used in dressings, for example. The composition isespecially advantageous for wound treatment, since sugar favors thehealing of wounds. Sugar also lowers the water activity, and thisprevents growth of microorganisms. However, sugar has to be concentratedin order to function as a preservative. When diluted with urine, forexample, it functions as a nutrient substance.

Lowering the pH can also inhibit the growth/activity of bacteria. Thiscan be done, for example, with acidic superabsorbent polymers, acidicpulp, the addition of acids, etc.

Absorbent articles in some cases comprise liquid-impermeable butvapor-permeable backing layers. Such backing layers allow gases to passthrough, such as air and water vapour, but are impervious to liquid suchas urine and menstrual fluid. In this way, the moisture content againstthe user's skin during use is lowered, and the product is felt to beairier. Liquid can come directly from the user's skin through seating,or it can come from excreted body fluid taken up in the absorbent core.The reduction in moisture through use of vapor-permeable layers has notbeen sufficient to reduce the growth of microorganisms. Foul odors canalso penetrate through a vapor-permeable layer.

Lactic acid bacteria in absorbent articles require special protectionduring storage, and a more stable solution is needed to the problem. Itwould also be desirable to find a solution that is more environmentallyfriendly and simpler than the earlier solutions outlined above.

The undesired presence of microorganisms in absorbent articles, even ata low concentration, can create possibilities for pathogenicmicroorganisms to increase in numbers in certain situations. The risk offoul odors in absorbent articles then increases, since undesiredbacteria often lead to the occurrence of bad odors. There is therefore aneed to prevent the occurrence and growth of microorganisms in absorbentarticles. The present invention is aimed at solving these problems.

SUMMARY

The object of the present invention is to provide an absorbent articlewith improved hygiene which reduces the risk of infections and of foulodors by limiting the growth and activity of undesired microorganisms.

This can be achieved, according to an exemplary embodiment of thepresent invention, by an absorbent article, such as a diaper, diaperpants, sanitary towel or incontinence protector, with vapor-permeablebacking layer, comprising at least one monovalent salt of the form X⁺Y″in a quantity of 1-75% by weight (% dry) calculated on the basis of theweight of the core.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an absorbent article according to the invention in the formof a sanitary towel.

FIG. 2 shows an absorbent article according to the invention in the formof an incontinence protector intended for men.

FIG. 3 shows an absorbent article according to the invention in the formof a sanitary towel, the crotch area being illustrated.

FIG. 4 is a diagram showing the growth of bacteria at different saltadditions.

FIG. 5 shows ammonia production at different salt additions.

FIG. 6 is a graph showing the rising salt concentration in combinationwith a vapor-permeable layer.

FIG. 7 shows water activity in 3% strength salt solutions.

FIG. 8 shows the water activity at different dilutions of salts inElga-H₂O.

DEFINITIONS

The inventors define two different ways of specifying the salt contentin a product. The first is % dry. % Dry describes the quantity of saltin relation to the weight of the core, as shown below. This descriptionis used by us for the dry product, i.e. when the product is produced andbefore it is used.

% dry=Ms/(Ms+Mk)

Ms=mass of the added salt

Mk=weight of the core, e.g. weight of SAP+weight of pulp+any otheringredients in the core.

The inventors also define a salt content in the wet product, namely %wet. % Wet describes the quantity of added salt (% dry) that dissolvesin the liquid when the product is used. The quantity of liquid variesdepending on the user's situation, and the liquid can be synthetic testliquid or real body fluids such as urine, menstrual fluid, etc.

% wet=Ms/Mv

Ms=mass of the added salt

Mv=mass of the liquid

The resulting salt content, and thus also the water activity in the wetproduct, is an important factor for whether and to what extent thebacterial growth is inhibited and hygiene thus improved. Therefore, %wet is an important parameter to use in describing the effect of addedsalt (% dry). When the product is wetted with urine, test liquid, etc.,salt is added with the inherent salt content of the liquids, but thishas not been calculated into the specification of % wet.

Water activity is a measure of the quantity of free water in a systemand is an important factor for whether microorganisms can grow or not.The definition for water activity is: A_(W)=P_(S)/P₀ where P_(S) is thewater vapor's partial pressure over the specimen at a certaintemperature, and P₀ is the water vapor's partial pressure over purewater—at the same temperature. The water activity is a dimensionlessnumber between 0 and 1, where the water activity for pure water is 1,and, when the water activity is 0, there is no free water.

“Pure salt” is understood as at least 99% by weight of salt.

“Wet zone” is to be understood as the zone of the absorbent article thatreceives the body fluid. This zone is often placed at the center of theabsorbent article. If the article has a front part, a rear part and acrotch part, the crotch part is often the part of the absorbent articlethat first receives body fluid. In the case of an incontinence protectordesigned for men, the wet zone is essentially the part of the articlethat receives urine from the penis, and this is usually the central partof the article. Since an absorbent article can be placed differently onthe user's body and can differ from case to case, reference is also madeto an “intended wet zone”. This is the zone that is intended to functionas wet zone during use of the absorbent article, and this is of coursealso the central part, as mentioned above.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to an absorbent article 1, such as adiaper, diaper pants, sanitary towel or incontinence protector, with avapor-permeable backing layer, FIG. 1 showing a sanitary towelcomprising a liquid-permeable cover layer intended to be directed towardthe user during use, a liquid-impermeable but vapor-permeable backinglayer intended to be directed away from the user during use, and anabsorbent core between the cover layer and the backing layer, saidarticle having a longitudinal direction 2, a transverse direction 3, twosubstantially longitudinal side edges 4, 5, a substantially transversefront edge 7, a substantially transverse rear edge 8, a substantiallylongitudinal midline 6, a front part 9 and a rear part 10 on each sideof a substantially center line 11, said midline 6 and center line 11intersecting one another at a point of intersection 12, and theabsorbent article comprises at least one monovalent salt of the formX⁺Y⁻ in a quantity of 1-75% by weight (% dry) calculated on the basis ofthe weight of the core. When liquid such as urine is added to theabsorbent article, the water activity in the absorbent articleincreases. When the salt is present in an absorbent article, it willlead to a lowering of the water activity in the absorbent article whenthe liquid has come into contact with the salt. The article preferablycontains 5-55% by weight (% dry) of salt calculated on the basis of theweight of the core. The transverse center line 11 is preferably placedsubstantially at the midway point of the length of the article.

During use, water vapor is driven out through thevapor-permeable/breathable backing layer. The driving force of the watervapor out through the backing layer will of course be affected by theenvironment on the outside of the backing layer. Water vapor on theinside is more easily conveyed outward if the moisture content on theoutside of the article is low. When water vapor is conveyed out throughthe layer, the result is that liquid disappears from the absorbentarticle, but the salt remains in the article. This leads to a rise inthe salt concentration in the excreted liquid remaining in the article,and this in turn leads to a further drop in the water activity, whichfurther reduces the bacterial growth. When the bacterial growthdecrease, there is the further advantage that undesired odors areinhibited. The disadvantage of a vapor-permeable backing layer allowingundesired odors to pass through can thus be eliminated. A drop intemperature is also achieved as the vapor is driven off, and thebacteria will then grow more slowly. This provides better comfort andhygiene.

Using salt in an absorbent article is simple and inexpensive. It mayalso be more environmentally friendly than using other antimicrobialagents, it can be good for the skin, and it does not make bacterialstrains resistant. The salt thus lowers the water activity in a wetabsorbent article. A wet absorbent article is also to be understood asan article that is moist with excreted material from the user's genitalarea or skin. The salt coordinates water and thereby reduces wateractivity, which inhibits microbial growth during use. This is anadvantage in hygiene terms, since it, for example, reduces the risk ofinfections. The reduced microbial growth also functions as anodor-inhibitor, since microorganisms, such as bacteria for example,produce malodorous substances, for example ammonia. The quantity ofammonia produced is decreased by virtue of the invention.

The combination with a vapor-permeable backing layer is thus extremelyeffective, since bad odors are not generated to such a great extent inthe article, and there is therefore a lesser quantity of bad odors thatcan pass through the backing layer. This effect is intensified as thewater activity is lowered by way of the vapor-permeable backing layer.

The quantity of free water, the water activity, is a very importantparameter that determines whether bacteria, yeast or mold can grow invarious environments. In absorbent articles, it is primarily bacteriathat grow. Bacteria are the group of microorganisms that is firstinhibited when the water activity decreases.

The backing layer is liquid-impermeable, but vapor-permeable, in orderto allow water vapor to pass through from the article. This isespecially advantageous if it is a large article, since it provides alarger surface area for escape of vapor.

X⁺ is chosen from among Na⁺, K⁺, [[NH₄ ⁺]] NH₄ ⁺ , and Y⁻ is chosen fromamong C₂H₃O₂ ⁻ and C₃H₅O₂ ⁻. The monovalent salts can be chosen fromamong NaCl, NaC₂H₃O₂ (Na acetate), NaC₃H₅O₂ (Na propionate), KCl,KC₂H₃O₂ (K acetate), KC₃H_(S)O₂ (K propionate), NH₄Cl, NH₄C₂H₃O₂ andNH₄C₃H₅O₂. These monovalent salts perform well in reducing the wateractivity.

Polyvalent ions also work, but polyvalent positive ions can have anegative effect on the superabsorbent polymer. By ion bonding,polyvalent positive ions can interact with the superabsorbent polymernetwork and negatively affect the swelling capacity and thus itsabsorption, more negatively than do the monovalent salts.

The salt is preferably distributed within a central area around thepoint of intersection 12. This area is regarded as a wet zone, since itis the area that urine and excreted body fluids will first arrive at onthe absorbent article. The salt is thus preferably distributed in theintended wet zone.

It is advantageous to place the salt in and around the area round thewet zone, since the salt needs to be in contact with the liquid that isapplied to the absorbent article in order to be able to reduce the wateractivity to any extent. The size of the wet zone depends on whichabsorbent article is intended, how much liquid the user excretes, etc. Adiaper for example has a larger wet zone than a sanitary towel.Moreover, the wet zone for a relatively large incontinence protectorwill be larger than for a small incontinence protector. In the wet zone,the salt is dissolved in the liquid and is then transported with theliquid to other parts of the article.

The area within which the salt is distributed extends at least 1.5 cmfrom the center line 11 in the longitudinal direction 2 and at least 1.5cm from the midline 6 in the transverse direction 3. The area extends inboth directions from the center line and the midline. This is an areawhich to a large degree will receive liquid from the user when thearticle is applied. To make the best use of it, the salt should beplaced in this area. The salt can also be placed in an area that coversthe whole of the absorbent article. It will then not come into contactwith liquid everywhere, but it may be simpler to add the salt withoutrestricting the positioning thereof.

According to one embodiment of the present invention, the absorbentarticle is an incontinence protector intended for men, and it isintended to be placed such that it covers the male member. Anincontinence protector 21 for men is shown in FIG. 2. Like the absorbentarticle in FIG. 1, the article in the form of an incontinence protectorfor men in FIG. 2 has a liquid-permeable cover layer intended to bedirected toward the user during use, a liquid-impermeable backing layerintended to be directed away from the user during use, and an absorbentcore between the cover layer and the backing layer, said article havinga longitudinal direction 22, a transverse direction 23, twosubstantially longitudinal side edges 24, 25, a substantially transversefront edge 27, a substantially transverse rear edge 28, a substantiallylongitudinal midline 26, a front part 29 and a rear part 210 on eachside of a substantially transverse center line 211. The midline 26 andcenter line 211 intersect one another at a point of intersection 212.The incontinence protector also has fastening arrangements 218, 219. Thesalt is distributed within a central area around the point ofintersection 212. In the same way as above, the salt is distributedwithin an area that extends at least 1.5 cm from the center line 211 inthe longitudinal direction 22 and at least 1.5 cm from the midline 26 inthe transverse direction 23.

Some absorbent articles also comprise a crotch part. Such an article isshown in FIG. 1 and in FIG. 3. The crotch part is not shown in FIG. 1,but is illustrated in the absorbent article 31 shown in FIG. 3. Like theabsorbent article in FIG. 1, the article has a liquid-permeable coverlayer intended to be directed toward the user during use, aliquid-impermeable backing layer intended to be directed away from theuser during use, and an absorbent core between the cover layer and thebacking layer, said article having a longitudinal direction 32, atransverse direction 33, two substantially longitudinal side edges 34,35, a substantially transverse front edge 37, a substantially transverserear edge 38, a substantially longitudinal midline 36, a front part 39and a rear part 310 on each side of a substantially transverse centerline 311. The midline 36 and center line 311 intersect one another at apoint of intersection 312. The crotch part 320 is placed substantiallyat the center of the absorbent article. The center line 311 passesthrough the crotch part 320, which has a length of about 7-12 cm. Thecrotch part is delimited in the figure by a front, substantiallytransverse border 321 and by a rear, substantially transverse border322. Absorbent articles with a crotch part are, for example, sanitarytowels, diapers and incontinence protectors, but not incontinenceprotectors intended for men, where the protection is intended to bepositioned such that it essentially covers the male member. The crotchpart is the area that receives excreted body fluids and can define a wetzone, and salt is distributed preferably in a crotch part of anabsorbent article.

In some cases it is advantageous if a larger area of the article is wet.For example, for products having the ability to take up a lot of liquid,it is expedient if the liquid is distributed over a large area of thearticle. A larger amount of vapor will then escape through thevapor-permeable backing layer. It can then be advantageous to have aspreader layer in the absorbent article, which layer spreads the liquidin the directions outward from the wet zone. The greater the area withinwhich the liquid is spread, the better the escape of vapor through thevapor-permeable backing layer. The absorbent article thus preferablycomprises a spreader layer.

A spreader layer can comprise a material having a capillary forcegreater than the adjacent material from which it will receive liquid.The spreader layer will also have the ability to quickly suck liquid bya capillary effect along the plane of the layer. A spreader layer can beproduced by the wet-laying principle, by which means a dry paper layerwith wet strength is obtained. It is also possible to produce a spreaderlayer using the air-laying principle. For a dry-laid structure toexhibit good strength, binders such as glue or melt fibers should beadmixed. The spreader layer can include a mixture of one or moredifferent types of cellulose fibers, non-cellulose fibers, such as wool,silk, or synthetic fibers. Compressed chemical pulp can be used, as cannonwoven material, for example spunlace material, meltblown material andthe like.

The spreader layer can be arranged between the cover layer and theabsorbent core or between the absorbent core and the backing layer.Moreover, the spreader layer can be arranged within the absorbent core,for example between two absorption sheets.

For some products, however, it is not advantageous to include a spreaderlayer. This applies, for example, to articles intended for a smallquantity of liquid.

The salt does not need to be distributed in the whole area that theliquid will cover. The liquid dissolves the salt and will then transportthe salt with it.

The salt can also be placed in an area that covers the whole absorbentarticle. It will then not come into contact with the liquid everywhere,but it can be simpler to add the salt without restricting thepositioning thereof.

The salt can be distributed within the absorbent core. This is the placewhere liquid is collected, and it is important that the salt comes intocontact with the liquid collected in the absorbent core.

The salt can also be distributed on top of the absorbent core. There, itcomes directly into contact with incoming liquid as soon as the liquidhas passed through the cover layer and any other layers arranged betweenthe cover layer and the absorbent core. The salt is dissolved and canthen be transported down into the absorbent core.

The absorbent article comprises a vapor-permeable backing layer whichcan have a vapor permeability of at least 500 g/m² and 24 h, measured at23° C. according to the ASTM-6701 MDCON method. Such vapor permeabilityis needed so that vapor will be able to pass outward through the backinglayer in a satisfactory way and so that it can give an effect on thesalt content.

The salt can have a particle size of 50 to 1500 μm. The salt can be inthe form of grains, and it is the diameter of the grains that is stated.Since the salt grains can be angular shaped and can be irregular inshape, the diameter concerned is an average diameter. The salt can alsobe in the form of some other particle shape, such as flakes. The size ofthe flakes refers to the extending size of the flakes, i.e. the width ofthe flakes, and can be up to 1 cm. The flakes can have a thickness of upto about 5 mm. The size can depend on which article they are placed in,where in the article the salt is placed, and whether a rapid or slowdissolution of the salt is wanted. If the salt is placed on top of theabsorbent core, it may be expedient to have relatively small grains.Liquid quickly comes into contact with the salt, and the liquid willpass quite quickly down into the absorbent core and is probably not incontact with the solid salt for any long time. If the salt is placedunder the absorbent core, the liquid has already passed through the coreand will not be transported farther downward in the article. Smallgrains or particles may be expedient if the salt does not come intocontact with as much liquid (since the liquid does not pass through thesalt) as it does when the salt is placed within or on top of the core.At the same time, large grains may also be suitable, since the liquidand the salt can be in contact for a relatively long time.

As has already been mentioned, the salt is preferably placed in acentral area around the point of intersection of the midline and thecenter line, regardless of which layer the salt is placed in or on orunder. In addition, the absorbent core can be divided into severalabsorbent sheets and the salt can be placed between these sheets.Moreover, the salt can be placed in one of several sheets or in all thesheets of an absorbent core. Further layers, such as an airlaid layer,arranged between cover layer and absorbent core, can also include thesalt. Moreover, a layer can be arranged under the absorbent core,between the core and the backing layer where the salt is placed.

The salt can be added as pure salt, i.e. with a purity of 99%. Ittherefore does not really contain any additives. The salt is aninexpensive constituent of absorbent articles, and it is also easy toapply it to the article. The salt can also be applied by spraying on ofa salt solution, or a layer of the absorbent article can be impregnatedwith or immersed in a salt solution, and the salt solution is thenallowed to evaporate such that the solvent in the form of water, forexample, is dried off from the article. The salt will then be present inthe form of salt crystals in the absorbent article. The salt ispreferably in pure form, i.e. with a purity of 99% by weight of salt.

The salt can be applied in the form of particles and mixed in duringforming of the absorbent core. The salt grains can of course also beapplied in distinct layers under the core, inside the core or on top ofthe core. If the salt is dissolved in liquid or suspended in a liquid,the salt solution or the salt suspension is applied to the absorbentcore by spraying. The spraying is followed by a drying step. It isadvantageous if the salt solution/salt dispersion/salt suspension isapplied already by the manufacturer, since the spraying step can beavoided during the preparation of the article. Salt solution can besprayed onto one or both sides of an absorbent core.

The solvent used for the salt can be water. A volatile organic solventsuch as ethanol can be used as dispersant for the salt, or a mixture ofa water-miscible organic solvent such as ethanol. The ethanol willfunction as a carrier for the salt.

The undesired bacteria whose growth is prevented are very sensitive to adecreased water activity. E. coli and Proteus reduce their growth evenat a water activity of 0.98 and have great difficulty in growing below awater activity of 0.95.

There are several contributory factors for the reduced water activity inan absorbent article. Urine has a salt content of about 1%, which alsogives a decrease in the water activity. The salt content in urine variesgreatly from person to person and from case to case (depending, amongother things, on how much has been drunk). The water activity will thendepend on the amount of added salt, on which salt has been added, onwhat quantity of urine is conveyed to the article, and on the urine'sinherent salt content.

The salt can be added in a quantity such that the water activity in theabsorbent article is under 0.98. A lower limit for the quantity of saltthat has to be added is, for example, 10-30% by weight calculated on thebasis of the weight of the core and an incontinence product with a highload. Even such a low content as 1% by weight can in some products givea water activity of under 0.98. Higher salt contents and, consequently,lower water activity provide better inhibition of bacterial growth. Thesalt content of urine will also contribute to the decrease in wateractivity. An upper limit of 75% by weight (% dry) is directed primarilyfor reasons of production engineering.

It has been found that a salt content of about 3% by weight (% wet) insynthetic urine is very advantageous for decreasing the water activity,for reducing the production of ammonia and for inhibiting bacterialgrowth in absorbent articles. Various tests have been carried out for 1,2 and 3% by weight (% wet) of added salt in synthetic urine as aredescribed below under the examples. It is therefore advantageous, in anabsorbent article that is wet, to have a salt concentration of about 3%by weight (% wet) of added salt to synthetic urine. 1 and 2% by weight(% wet) of added salt also function well.

An absorbent article has a maximum absorption capacity that differs fordifferent products. For the quantity of added salt to be 3%, or 2 or 1%(% wet), when wetted during use of an absorbent article, differentquantities of salt therefore have to be added, depending on whichproduct is involved. The products have different weights, differentweights of the core, and different maximum absorption capacity. To reach3% by weight (% wet) of salt in the liquid in an absorbent article, themaximum absorption capacity has been calculated, i.e. the maximum weightof liquid that an absorbent article is able to absorb, and from this itis possible to calculate how much salt needs to be added to theabsorbent article to reach a salt content of about 3% by weight (% wet).An absorbent article is thus intended in which the quantity of saltadded corresponds to 3% by weight (% wet) of the maximum weight ofliquid that the article can absorb, for example 3 g of salt are added ifthe absorption capacity is 100 ml. Since urine normally has an inherentsalt content of about 1% by weight, the final content is therefore about4% by weight.

The tests shown in the examples were all carried out with maximum load(the greatest quantity of liquid that the product can absorb) of theproducts. This corresponds to the least favorable situation, i.e. thedilution of the salt is at its greatest. In real use situations, amaximum load is rarely reached before the product is changed, i.e. thesalt concentration is higher and the effect better.

Weights for different products now follow here. Examples are also givenof how the maximum absorption capacity is calculated. The quantity ofsalt that has to be added can in this way be easily calculated for therespective products. The dry weight for the core in a female panty lineris 1-3 g, for sanitary towels 3-15 g, for infants' diapers 20-50 g, andfor incontinence products 10-120 g (for the simplest to the heaviestprotector).

The maximum absorption weight for chemical fluff pulp is usually givenas 6 ml/g and for superabsorbent polymers it is usually given as 25ml/g. An incontinence product with 54 g (divided into two layers of 17and 34 g) of chemical fluff pulp and 18.5 g of superabsorbent polymerwill then have a maximum absorption capacity of 787 ml, i.e. a maximumabsorption of 324 ml for the chemical fluff pulp, and 463 ml forsuperabsorbent polymer. If such an article is to have a quantity ofadded salt corresponding to 3% by weight (% wet) of the absorptioncapacity volume, this will be about 24 g of salt.

An incontinence product with 47 g (divided into two layers of 14 g and33 g) of chemical fluff pulp and 6 g of superabsorbent polymer will havea maximum absorption capacity of 432 ml. 3% by weight (% wet) of addedsalt calculated on the basis of the maximum absorption capacity is thenabout 13 g of salt. These calculations can be carried out for variousproducts by a person skilled in the art, taking account of thecomposition of the core and its maximum absorption capacity.

If the salt is placed in different zones in the product, theconcentration may be higher in certain areas. When the product is notmaximally loaded, the concentration is also higher. And when theconcentration rises, the effect is better.

When vapor can be transported out through the vapor-permeable backinglayer, the salt concentration will increase in the absorbent article.One example below shows that the salt concentration during one night fora night diaper with vapor-permeable backing layer can be increased from2 to 3% by weight of salt, which can be seen in FIG. 6. This is animportant change, since the quantity of bacteria and the development ofammonia decreases when the salt concentration is increased from 2 to 3%by weight, as can be seen in FIGS. 4 and 5.

E. coli, P. mirabilis and E. faecalis are chosen as relevant testbacteria in the examples. They are all examples of bacteria that onedoes not wish to have growing in large numbers in incontinence products,for example, during use. They can all cause urinary tract infections(UTI), for example. E. coli is often reported as the most common causeof UTI. Proteus is also urease-positive, which means that it can cleaveurea to ammonia. Ammonia is an important cause of foul odors in usedincontinence products.

For 3% added salt (% wet) (NaCl high in FIG. 4), the growth of all threetest bacteria after 12 hours will lie below log 5.5. This is a greatdifference compared to the reference specimen, and a considerablehygiene improvement.

The vapor-permeable backing layer will also contribute to the wateractivity being kept down. In addition, it is possible to usevapor-permeable backing layers with greater vapor permeability, sincebad odors from the absorbent article are reduced.

There now follow examples of different materials from which an absorbentarticle according to the invention can be made.

The cover layer can be made of a woven material, a nonwoven material, apolymer material such as perforated plastic films, porous foam, orreticulated foam. Suitable woven and nonwoven materials can includenatural fibers (e.g. cellulose or cotton fibers), synthetic fibers (e.g.polymer fibers, such as polyesters, polypropylene or polyethylene) or acombination of natural and synthetic fibers. Nonwoven materials can beproduced in a number of different ways, such as spunbond, carded,wet-laid, meltblown, hydroentangled, and combinations of the differentmethods.

The vapor-permeable backing layer is often composed of a plastic film,for example produced from polyethylene, polypropylene or a laminate ofthese. By addition of a filler, e.g. titanium dioxide, chalk or thelike, the layer is made vapor-permeable. The layer can also bestretched. It is also possible to use a liquid-permeable material thatis treated such that is becomes liquid-impermeable. Such treatments caninvolve the surface being partially coated with a liquid-impermeableglue, the surface being coated with a hydrophobic material, theliquid-permeable layer being laminated together with a liquid-tightmaterial, an initially liquid-permeable material being hot-calenderedsuch that the surface melts and the material thus becomesliquid-impermeable. The coating is not applied to the whole surface;instead, pores remain through which vapor can pass. A vapor-permeable,but liquid-impermeable material can also include liquid-impermeable,vapor-permeable textile materials or laminates including a number oftextile layers or layers of film and textile material. Examples ofsubstantially liquid-impermeable but vapor-permeable textile materialsare hydrophobic nonwovens, porous plastic films, foamed material withfilled cells, laminates composed of one or more layers of spunbondcombined with meltblown layers, filled plastic film, waxed materiallayers and treated wadding.

The backing layer can have an adhesive attachment in the form of beadsof adhesive, for example, on that side of the backing layer facing awayfrom the cover layer, to enable it to be secured in panties, underpantsor knickers. A release material may be applied on top of the adhesive inorder to protect the adhesive when the product is not in use.

The absorbent core can also be composed of one or more layers ofcellulose fibers, for example cellulose fluff pulp, airlaid,dry-defibered or compressed pulp. Other materials that can be usedinclude, for example, absorbent nonwoven material, foam material,synthetic fibre material or peat. Apart from cellulose fibers or otherabsorbent materials, the absorbent core can also comprise superabsorbentmaterials, superabsorbent polymers, which are materials in the form offibers, particles, granules, films or the like. Superabsorbent polymersare inorganic or organic materials which are capable of swelling inwater and are insoluble in water and which have the capacity to absorbat least 20 times their own weight of an aqueous solution containing0.9% by weight of sodium chloride. Organic materials that are suitablefor use as superabsorbent polymers can include natural materials such aspolysaccharides, polypeptides and the like, and also synthetic materialssuch as synthetic hydrogel polymers. Such hydrogel polymers can include,for example, polyacrylic acid, alkaline metal salts of polyacrylicacids, polyacrylamides, polyvinyl alcohol, polyacrylates,polyacrylamides, polyvinyl pyridines and the like. Other suitablepolymers include hydrolyzed acrylonitrile-grafted starch, acrylicacid-grafted starch, and isobutylene maleic acid anhydride copolymersand mixtures thereof. The hydrogel polymers are preferably slightlycross-linked to ensure that the material remains essentially insolublein water. Preferred superabsorbent materials can be surface cross-linkedso that the external surface or shell of the superabsorbent particle,fiber, sphere, etc., has a higher cross-linking density than the innerpart of the superabsorbent. The proportion of superabsorbents in anabsorbent core can be between 10 and 90% by weight, or preferablybetween 30 and 70% by weight.

The absorbent core can comprise layers of different materials withdifferent characteristics as regards their ability to receive liquid,their liquid distribution capacity and storage capacity. The absorbentcore is in most cases extended in the longitudinal direction and can,for example, be rectangular, T-shaped or hourglass-shaped. Anhourglass-shaped core is wider in the front and rear parts than in thecrotch part, in order to provide effective absorption, at the same timeas the design makes it easier for the product to be shaped close to andaround the wearer, thus providing a better fit around the legs.

In addition, the absorbent article can include a transport layer betweenthe cover layer and the absorbent core. The transport layer is a porous,flexible material and can comprise one or more of the following:airlaid, wadding, tissue, carded fiber web, superabsorbent particles orsuperabsorbent fibers. A transport layer has a high instantaneouscapacity to receive liquid and is able to store liquid temporarilybefore it is absorbed by the underlying absorbent core. The transportlayer can cover all or parts of the absorbent core.

The cover layer, the backing layer and any intermediate materials aresealed at the edges of the product, which can be done by thermalsealing, for example, or by some other conventional means.

The absorbent article can also comprise wings on its sides. It can alsocomprise elastic in order to provide better contact with the body whenthe product is being worn, and also to reduce leakage.

Salt has thus been found to function extremely well in inhibiting thegrowth of bacteria in absorbent articles. Absorbent articles oftencomprise superabsorbent materials, and it is known that salt has anegative impact on many superabsorbent polymers, although there do existsuperabsorbent polymers that are insensitive to salt. A monovalent salt,however, has little impact on them, and the positive effect oninhibition of bacteria outweighs this consideration in these cases. Amonovalent salt is advantageous if superabsorbent materials are used inthe article. The absorbent articles with bacterial inhibition are easyto produce. The salt in itself is inexpensive and environmentallyfriendly. Together with the vapor-permeable backing layer, positiveeffects are achieved, as outlined above, and are intensified as the saltconcentration increases during use of the absorbent article. At the samesalt content and with the same frequency of changing the article,enhanced hygiene is achieved, and since the temperature drops in thearticle when vapor escapes through the backing layer, enhanced comfortis achieved and less growth of microorganisms. A breathable productwhich is comfortable for the user is obtained and, in addition, the riskof infection during use is reduced.

The invention will now be illustrated by the following examples.

EXAMPLES

Test liquid 1 is used for pH measurements, bacterial growth measurementsand ammonia measurements (referred to in method 2): Sterile, syntheticurine to which a growth medium for microorganisms has been added. Thesynthetic urine contains monovalent and divalent cations and anions andurea and has been produced in accordance with the information in Geigy,Scientific Tables, vol. 2, 8th ed., 1981, page 53. The growth medium forthe microorganisms is based on the information of Hook and FSA mediumfor enterobacteria. The pH in this mixture is 6.6.

Test liquid 2 is used for water activity measurements:

Recipe—Synthetic Urine

Magnesium sulfate 0.66 g/l (100 ml stock solution for 5 liters)

Potassium chloride 4.47 g/l (100 ml stock solution for 5 liters)

Sodium chloride 7.60 g/l (38.0 g for 5 liters)

Urea (carbamide) 18.00 g/l (90.0 g for 5 liters)

Potassium dihydrogen phosphate 3.54 g/l (100 ml stock solution for 5liters)

Disodium hydrogen phosphate, anhydrous 0.745 g/l (100 ml stock solutionfor 5 liters)

Triton X-100, 0.1% strength 1 m00 g/l (5.0 g for 5 liters)

Deionized water to 1 l(5.0 l for 5 liters)

New coccine (dye) 10% strength 0.4 g/l (2.0 g for 5 liters)

Principle

The four stock solutions are prepared first. Chemicals and stocksolutions are mixed to give the solution for use.

Implementation

Weighing of the chemicals and preparation of the test liquid requireaccuracy so that the solution for use will have the correct properties.Prepare the stock solutions according to the recipes. Mix the solutionof chemicals and stock solutions in the sequence stated in the recipe.

Stock Solutions (Storage Time 1 Month)

0.274 M magnesium sulfate: dissolve 33 g of magnesium sulfate indeionized water to give 1 l. (To 5 l of stock solution: 165 g MgSO₄)

2.998 M potassium chloride solution: dissolve 223.5 g of potassiumchloride in deionized water to give 1 l. (To 5 l of stock solution:1,117.5 g KCl)

1.301 M potassium dihydrogen phosphate solution: dissolve 177 g ofpotassium dihydrogen phosphate in deionized water to give 1 l. (To 5 lof stock solution: 885 g KH₂PO₄)

0.262 M sodium dihydrogen phosphate solution: dissolve 37.25 g ofdisodium hydrogen phosphate in deionized water to give 1 l. (To 5 l ofstock solution: 186.25 Na₂HPO₄)

Solution for Use (Storage Period 3 Weeks)

Fill flask to 60% with deionized water

Add the magnesium sulfate solution

Add the potassium chloride solution

Add the sodium chloride

When it is dissolved, add the urea

When it is dissolved, add the potassium dihydrogen phosphate solution

Add the disodium hydrogen phosphate solution

Add Triton X-100

Fill with deionized water to the exact quantity

Physical Properties

The solution for use has the following values:

Surface energy (surface tension) 60±3 mN/m

Conductivity 23±2 mS

pH 6.0±0.5

Temperature 22±2° C.

Method 1: Production of Absorbent Test Specimens for Testing

Absorbent test specimens were punched out from an absorbent coreproduced in a test plant. A standard method for mat-forming of a corewas used during production of the core in the test plant. The absorbenttest specimen included a homogeneous mixture of fluff pulp, 0.72 gWeyerhauser pulp (NB 416) and 0.48 g of superabsorbent polymer (SAP)(Degussa SXM9135). The absorbent core, was compressed to a bulk ofaround 8-10 cm³/g. The size of the punched-out test items was 5 cm indiameter, the weight was about 1.2 g.

Method 2: Measurement of Ammonia Formation in Absorbent Cores

Absorbent test specimens were produced according to method 1. Abacterial suspension of Proteus mirabilis was cultured in nutrientsolution at 30° C. overnight. The grafted cultures were diluted withtest liquid 1 and the quantity of bacteria was determined. The finalculture contained approximately 10⁵ organisms per ml of test liquid. Theabsorbent core was placed in a plastic tub and test liquid 1 containingbacteria was added to the absorbent core, after which the container wasincubated at 35° C. for 4, 6 and 8 hours, and then the specimens wereremoved from the container using a hand pump and a so-called Dragertube. The ammonia content was obtained as a color change on a scalegraduated in ppm or percent by volume.

Method 3: Measurement of Bacterial Growth in Absorbent Cores

16 ml (corresponding to a maximum absorption capacity of a test specimenaccording to method 1) of test liquid 1 containing the bacteria wereadded to a test specimen placed in plastic tubs, and a lid was fitted onthe tubs. The tubs were turned upside down and incubated in a warmcabinet at 35° C. After incubation for 0, 6 and 12 hours, the testsamples were placed in a plastic bag with peptone water and the contentwas homogenized (agitated and worked up) in a stomacker for 3 minutes.The homogenate was diluted in dilution tubes with peptone water and amicrobiological culture was spread on agar plates. Slanetz Bartley agarwas used for E. faecalis, and Drigalski agar for E. coli and P.mirabilis. The specimens were incubated at 35° C. for 1-2 days beforethe colonies were counted and the log CFU/ml calculated. Control testswere also carried out with absorbent cores without NaCl.

Method 4: Measurement of Water Activity

Water activity is measured as follows. The water activity meter is fromAqua Lab, Model Series 3 TE, Pentagon Devices Inc. (U.S. Pat. No.5,816,704). A specimen, a solution with different additions of differentsalts in Elga-H₂O or synthetic urine, test liquid 2. was placed in aplastic cup. The quantity is not critical, but it is important that thebottom surface of the plastic cup is covered. The specimen is moved intothe test chamber, which is closed, and the measurement commences. Whenthe test value is stable, a green light flashes and the water activitycan be read off digitally. The salts that were tested were: NaCl, KCl,NH₄Cl, KC₂H₃O₂ (K acetate), Na C₂H₃O₂ (Na acetate).

Example 1 Bacterial Growth on Addition of NaCl

Bacteria were cultured in nutrient broth and diluted to the desiredconcentration of about Log 3.3 in test liquid 1 (method 4). Absorbenttest specimens were produced according to method 1. 29 (high), 22(medium) and 12 (low) % by weight NaCl (% dry) were added to the core,corresponding to 3 (high), 2 (medium) and 1 (low) % by weight (% wet) inthe wet product. This proportion also applies to the other examples. Thebacterial growth was measured according to method 3.

The result is shown in FIG. 4, which clearly illustrates that the growthof all 3 test bacteria is lower after 6 and 12 hours, compared with thecontrol tests, but also that the inhibition is better with a higher saltcontent.

Example 2 Development of Ammonia on Addition of NaCl or CaCl₂

Absorbent test specimens were produced according to method 1. 16 ml,maximum absorption capacity, of test liquid 1 containing bacteria wereadded to a test specimen. 29 (high), 22 (medium) and 12 (low) % byweight of NaCl (% dry) and 22 (medium) % by weight of CaCl₂ (% dry) wereadded to the core. The quantity of developed ammonia was measuredaccording to method 2 after 6 and 8 hours. Control tests were alsocarried out using absorbent test specimens without NaCl or CaCl₂.

The results are shown in FIG. 5. The quantity of developed ammonia waslower for all the specimens with added salt compared to the controlspecimens. The lowest development of NH₃ was with the specimencontaining 29% by weight of NaCl.

Example 3 Measurement of Salt Concentration in Combination withVapor-Permeable Backing Layer

Calculations were based on a product from which a specimen was punchedout, the specimen had a diameter of 5 cm and included a cover materialof polypropylene nonwoven, superabsorbent particles (SAP) mixed withcellulose fluff and a backing layer of polyethylene Mocon 8000 g/m² and24 h, which corresponds to 333 g/m² and h. A more realistic vaporpermeability value was measured at 250 g/m² and h, using a vapometer(SWL 3 NO211 from Delfin Technologies Ltd.) The measurement was carriedout on wet product for 1 h with the vapometer.

16 ml of test liquid 2 were added to the diaper specimen containingdifferent amounts of added salt. The surface area of the diaper specimenwas 1,963×10⁻³ m² and had an approximate value of 0.5 g/h evaporation.0.48 g, 0.32 and 0.16 g of NaCl were added to the specimens 1, 2 and 3,respectively. The increase in the salt concentration is shown in FIG. 6.

For test 2, the salt concentration was changed from 2 to just over 3% byweight (wet %) for 12 hours, which can correspond to one night's use ofa diaper. A salt concentration increase from 2 to 3% by weight (wet %)leads to a reduction in the development of ammonia, which can be seen inFIG. 5, and a reduced growth of bacteria, which can be seen in FIG. 4.For all the salt concentrations that were examined, the saltconcentration increased when measured at 6 hours and at 12 hours.

Example 4 Measurement of Water Activity on 3% Strength Salt Solutions

The water activity was measured in Elga-H₂O (USF Elga, distilled water)and synthetic urine, test liquid 2. To show that additions of salt leadto a reduction in water activity, the water activity for different saltswas measured in Elga water and test liquid 2. On measurement of thewater activity in test liquid 2 with its inherent salt content, a wateractivity of 0.984 was measured. For Elga water, the water activity wasmeasured to 0.999.3% by weight of various different salts were added totest liquid 2 and Elga water and the water activity was measuredaccording to method 4.

Clear decreases in water activity are shown in FIG. 7. The wateractivity after addition of 3% salt in test liquid 2 is below 0.975.

Example 5 Water Activity at Different Concentrations of NaCl, Naacetate, NH₄Cl

The water activity was measured according to method 4, but withdifferent concentrations of salt in Elga-H₂O. FIG. 8 shows clearly howthe water activity drops as the concentrations of salt increase.

1. An absorbent article, such as a diaper, diaper pants, sanitary towelor incontinence protector, with a vapor-permeable backing layer,comprising a liquid-permeable cover layer intended to be directed towardthe user during use, a liquid-impermeable but vapor-permeable backinglayer intended to be directed away from the user during use, and anabsorbent core between the cover layer and the backing layer, saidarticle including a longitudinal direction, a transverse direction, twosubstantially longitudinal side edges, a substantially transverse frontedge, a substantially transverse rear edge, a substantially longitudinalmidline, and a front part and a rear part on each side of asubstantially transverse center line, said midline and center lineintersecting one another at a point of intersection, wherein theabsorbent article comprises at least one monovalent salt of the formX⁺Y⁻ in a quantity of 1-75% by weight calculated on the basis of theweight of the core, wherein X⁺ is chosen from among Na⁺, K⁺, NH₄ ⁺, andY⁻ is chosen from among Cl⁻, C₂H₃O₂ ⁻ and C₃H₅O₂ ⁻.
 2. (canceled)
 3. Theabsorbent article as claimed in claim 1, wherein the salt is distributedwithin a central area around the point of intersection.
 4. The absorbentarticle as claimed in claim 1, wherein the area extends at least 1.5 cmfrom the center line in the longitudinal direction and at least 1.5 cmfrom the midline in the transverse direction.
 5. The absorbent articleas claimed in claim 1, wherein the vapor-permeable backing layer has avapor permeability of at least 500 g/m2 and 24 h.
 6. The absorbentarticle as claimed in claim 1, wherein the salt is distributed withinthe absorbent core.
 7. The absorbent article as claimed in claim 1,wherein the salt is distributed on top of the absorbent core.
 8. Theabsorbent article as claimed in claim 1, wherein the absorbent articlefurther comprises a spreader layer.
 9. The absorbent article as claimedin claim 8, wherein the spreader layer is arranged between the coverlayer and the absorbent core.
 10. The absorbent article as claimed inclaim 8, wherein the spreader layer is arranged between the absorbentcore and the backing layer.
 11. The absorbent article as claimed inclaim 8, wherein the spreader layer is arranged within the absorbentcore.
 12. The absorbent article as claimed in claim 1, wherein the salthas a particle size of 50 to 1500 μm.
 13. The absorbent article asclaimed in claim 1, wherein the salt is added in the form of flakes, andthe flakes have a width dimension of up to 1 cm.
 14. The absorbentarticle as claimed in claim 1, wherein the salt is added as pure salt.15. The absorbent article as claimed in any of the claim 1, wherein thesalt is added in a quantity such that the water activity in theabsorbent article is under 0.98 during wetting.
 16. The absorbentarticle as claimed in claim 1, wherein the salt is added in a quantitysuch that the concentration of each of E. coli, P. mirabilis and E.faecalis can be maintained below 6.5 Log CFU/ml in the article after 10hours of wetting with synthetic urine.